FRICTION
FRICTION
Y
ou might have seen a driver of a
car or a truck slowing down the
vehicle at a traffic signal. You, too,
slow down your bicycle whenever
needed by applying brakes. Have you
ever thought why a vehicle slows
down when brakes are applied? Not
only vehicles, any object, moving over
the surface of another object slows
down when no external force is
applied on it. Finally it stops. Have
you not seen a moving ball on the
ground stopping after some time?
Why do we slip when we step on a
banana peel (Fig. 9.1)? Why is it
difficult to walk on a smooth and
wet floor?
Fig. 9.1 : A boy falls down when he steps on a
banana peel
9.1 Force of Friction
Activity 9.1
Gently push a book on a table
[Fig. 9.2(a)]. You observe that it stops
after moving for some distance.
Repeat this activity pushing the
book from the opposite direction
[Fig. 9.2, (b)]. Does the book stop
this time, too? Can you think of an
explanation? Can we say that a force
must be acting on the book
opposing its motion? This force is
called the force of friction.
Fig. 9.2 (a), (b) : Friction opposes relative
motion between the
surfaces of the book and
the table
(a)
(b)
You will find the answers to such
questions in this chapter.
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You saw that if you apply the force
along the left, friction acts along the
right. If you apply the force along the
right, the friction acts along the left
direction. In both cases the force opposes
the motion of the book. The force of
friction always opposes the applied force.
In the above activity, the force of
friction acts between the surface of the
book and the surface of the table.
Is the friction the same for all the
surfaces? Does it depend on the
smoothness of the surfaces? Let us find
out.
9.2 Factors affecting Friction
Activity 9.2
Tie a string around a brick. Pull the
brick by a spring balance (Fig. 9.3).
You need to apply some force. Note
down the reading on the spring
balance when the brick just
begins to move. It gives you a
measure of the force of friction
between the surface of the brick
and the floor.
for this difference? Repeat this
activity by wrapping a piece of jute
bag around the brick. What do you
observe?
Fig. 9.3 : A brick is being pulled by spring
balance
Now wrap a piece of polythene
around the brick and repeat the
activity. Do you observe any
difference in the readings of the
spring balance in the above two
cases? What might be the reason
Spring Balance
Spring balance is a device used for
measuring the force acting on an
object. It consists of a coiled spring
which gets stretched when a force is
applied to it. Stretching of the spring
is measured by a pointer moving on
a graduated scale. The reading on the
scale gives the magnitude of the force.
Activity 9.3
Make an inclined plane on a smooth
floor, or on a table. You may use a
wooden board supported by bricks,
or books. [Fig. 9.4 (a)]. Put a mark
with a pen at any point A on the
inclined plane. Now let a pencil cell
move down from this point. How far
does it move on the table before
coming to rest? Note down the
distance. Now spread a piece of cloth
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Fig. 9.5 : Surface irregularities
over the table. Make sure that there
are no wrinkles in the cloth. Try the
activity again [Fig. 9.4 (b)].
Does the distance covered depend on
the nature of the surface on which the
cell moves?
Could the smoothness of the surface
of the cell also affect the distance
travelled by it?
(a)
(b)
Repeat this activity by spreading
a thin layer of sand over the
table. Maintain the same slope
throughout the activity.
In which case is the distance covered
the minimum? Why is the distance
covered by the pencil cell different every
time. Try to reason why? Discuss the
result.
Fig. 9.4 : The pencil cell covers different
distances on different surfaces
I shall try the activity
by wrapping a piece of
sandpaper around
the cell.
Friction is caused by the
irregularities on the two surfaces in
contact. Even those surfaces which
appear very smooth have a large number
of minute irregularities on them
(Fig. 9.5). Irregularities on the two
surfaces lock into one another. When
we attempt to move any surface, we have
to apply a force to overcome interlocking.
On rough surfaces, there are a larger
number of irregularities. So the force of
friction is greater if a rough surface
is involved.
Mark A
Mark A
Surface of book
Surface of table
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We see that the friction is caused by
the interlocking of irregularities in the
two surfaces. It is obvious that the force
of friction will increase if the two
surfaces are pressed harder. You can
experience it by dragging a mat when
nobody is sitting on it, and when a
preson is sitting on it.
friction and you find it somewhat easier
to move the box already in motion than
to get it started.
9.3 Friction : A Necessary
Evil
Recall now some of your experiences. Is
it easier to hold a kulhar
(earthen pot)
or a glass tumbler? Suppose the outer
surface of the tumbler is greasy, or has
a thin layer of cooking oil on it; would it
become easier or more difficult to hold
it? Just think : would it be possible to
hold the glass at all if there is no
friction?
Recall also how difficult it is to move
on a wet muddy track, or wet marble
floor. Can you imagine being able to walk
at all if there were no friction?
You could not write with pen or
pencil if there were no friction. When
your teacher is writing with chalk on
the blackboard, its rough surface rubs
off some chalk particles which stick to
Recall your experience when last time
you moved a heavy box from one place
to another (Fig. 9.6). If you have no such
experience, get that experience now.
What is easier — to move the box from
rest, or to move it when it is already in
motion?
The force required to overcome
friction at the instant an object starts
moving from rest is a measure of static
friction. On the other hand, the force
required to keep the object moving with
the same speed is a measure of sliding
friction.
When the box starts sliding, the
contact points on its surface, do not get
enough time to lock into the contact
points on the floor. So, the sliding
friction is slightly smaller than the static
Fig. 9.7 : A nail is fixed in the wall due to
friction
Fig. 9.6 : You have to push on the box to keep it
moving
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the black board. Could it happen if there
were no friction between the chalk and
the board?
If an object started moving, it would
never stop if there were no friction. Had
there been no friction between the tyres
of the automobiles and the road, they
could not be started or stopped or
turned to change the direction of
motion. You could not fix a nail on the
wall (Fig. 9.7) or tie a knot. Without
friction no building could be constructed.
Fig. 9.8 : Soles of shoes wear out due
to friction
Fig. 9.10 : Stricking a matchstick produces fire
by friction
produces heat. In fact, when a machine
is operated, heat generated causes much
wastage of energy. We shall discuss the
ways of minimising friction in the
following section.
Fig. 9.9 : Rubbing of your palms makes you
feel warm
On the other hand, friction is an evil,
too. It wears out the materials whether
they are screws, ball bearings or soles
of shoes (Fig. 9.8). You must have seen
worn-out steps of foot over-bridges at
railway stations.
Friction can also produce heat.
Vigorously rub your palms together for
a few minutes (Fig. 9.9). How do you
feel? When you strike a matchstick
against the rough surface, it catches fire
(Fig. 9.10).
You might have observed that the jar
of a mixer becomes hot when it is run
for a few minutes. You can cite various
other examples in which friction
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9.4 Increasing and Reducing
Friction
As you have seen in the previous section,
friction is desirable in some situations.
Have you ever thought why the sole
of your shoe is grooved [Fig. 9.11 (a)]? It
is done to provide the shoes better grip
on the floor, so that you can move safely.
Similarly, the treaded tyres of cars,trucks
and bulldozers provide better grip with
the ground.
In some situations, however, friction
is undesirable and we would want to
minimise it.
Why do you sprinkle fine powder on
the carrom board (Fig. 9.12)? You might
have noticed that when a few drops of
oil are poured on the hinges of a door,
the door moves smoothly. A bicycle and
a motor mechanic uses grease between
the moving parts of these machines. In
all the above cases, we want to reduce
Fig. 9.12 : Powder is sprinkled on the carrom
board to reduce friction
friction in order to increase efficiency.
When oil, grease or graphite is applied
between the moving part of a machine,
a thin layer is formed there and moving
surfaces do not directly rub against each
other (Fig. 9.13). Interlocking of
irregularities is avoided to a great extent.
Movement becomes smooth. The
substances which reduce friction are
called lubricants. In some machines, it
may not be advisable to use oil as
lubricant. An air cushion between the
moving parts is used to reduce friction.
We deliberately increase friction by
using brake pads in the brake system
of bicycles and automobiles. When you
are riding a bicycle, the brake pads do
not touch the wheels. But when you
press the brake lever, these pads arrest
the motion of the rim due to friction.
The wheel stops moving. Y
ou might have
seen that kabaddi players rub their
hands with soil for a better grip of their
opponents. Gymnasts apply some coarse
substance on their hands to increase
friction for better grip.
(a) (b)
Fig. 9.11 : (a) Soles of shoes and (b) tyres are
treaded to increase friction
Fig. 9.13 : Action of lubricant
Surface 1
Surface 2
Lubricant
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Activity 9.4
Take a few pencils which are
cylindrical in shape. Place them
parallel to each other on a table.
Place a thick book over it
(Fig. 9.15). Now push the book. You
observe the pencils rolling as the
book moves. Do you feel it easier to
move the book in this way than to
slide it? Do you think that
resistance to the motion of the book
has been reduced? Have you seen
heavy machinary being moved by
placing logs under it?
9.5 Wheels Reduce Friction
You must have seen attaches and other
pieces of luggage fitted with rollers. Even
a child can pull such pieces of luggage
(Fig. 9.14). Why is it so? Let us
find out.
Can we reduce friction to
zero by polishing surfaces
or using large amount of
lubricants?
Friction can never be
entirely eleminated. No
surface is perfectly smooth.
Some irregularities are
always there.
Fig. 9.15 : Motion of the book on rollers
When one body rolls over the surface
of another body, the resistance to its
motion is called rolling friction. Rolling
reduces friction. It is always easier to
roll than to slide a body over another.
That is the reason it is convenient to
pull luggage fitted with rollers. Can you
now understand why the wheel is said
to be one of the greatest inventions of
mankind?
Since the rolling friction is smaller
than the sliding friction, sliding is
replaced in most machines by rolling
by the use of ball bearings. Common
examples are the use of ball bearings
between hubs and the axles of ceiling
fans and bicycles (Fig. 9.16).
Fig. 9.14 : Rolling reduces friction
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9.6 Fluid Friction
You know that air is very light and thin.
Yet it exerts frictional force on objects
moving through it. Similarly, water and
other liquids exert force of friction when
objects move through them. In science,
the common name of gases and liquids
is fluids
. So we can say that fluids exert
force of friction on objects in motion
through them.
The frictional force exerted by fluids
is also called drag.
The frictional force on an object in a
fluid depends on its speed with respect
to the fluid. The frictional force also
depends on the shape of the object and
the nature of the fluid.
It is obvious that when objects
move through fluids, they have to
overcome friction acting on them. In
this process they lose energy. Efforts
are, therefore, made to minimise
friction. So, objects are given special
shapes. Where do you think scientists
get hints for these special shapes?
From nature, of course. Birds and
fishes have to move about in fluids
all the time. Their bodies must have
evolved to shapes which would make
them lose less energy in overcoming
friction. Look carefully at the shape
of an aeroplane (Fig. 9.17). Do you
find any similarity in its shape and
that of a bird? In fact, all vehicles are
designed to have shapes which reduce
fluid friction.
Fig. 9.17 : Similarity in shapes of an aeroplane and a bird
Fig. 9.16 : Ball bearings reduce friction
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KEYWORDS KEYWORDS
KEYWORDS KEYWORDS
KEYWORDS
BALL BEARING
DRAG
FLUID FRICTION
FRICTION
INTERLOCKING
LUBRICANTS
ROLLING FRICTION
SLIDING FRICTION
STATIC FRICTION
WHAT YOU HAVE LEARNT WHAT YOU HAVE LEARNT
WHAT YOU HAVE LEARNT WHAT YOU HAVE LEARNT
WHAT YOU HAVE LEARNT
Ü Friction opposes the relative motion between
two surfaces in contact. It acts on both the
surfaces.
Ü Friction depends on the nature of surfaces in
contact.
Ü For a given pair of surfaces friction depends
upon the state of smoothness of those
surfaces.
Ü Friction depends on how hard the two surfaces
press together.
Ü Static friction comes into play when we try to
move an object at rest.
Ü Sliding friction comes into play when an
object is sliding over another.
Ü Sliding friction is smaller than static friction.
Ü Friction is important for many of our activities.
Ü Friction can be increased by making a surface
rough.
Ü The sole of the shoes and the tyres of the
vehicle are treaded to increase friction.
Ü Friction is sometimes undesirable.
Ü Friction can be reduced by using lubricants.
Ü When one body rolls over another body, rolling
friction comes into play. Rolling friction is
smaller than sliding friction.
Ü In many machines, friction is reduced by
using ball bearings.
Ü Fluid friction can be minimised by giving
suitable shapes to bodies moving in fluids.
A riddle for you
In some situations, I oppose the motion
in other situations, I facilitate the motion
but, I always oppose
the relative motion
between two moving surfaces.
Put some lubricant, and
I become small there.
Make the moving surfaces rough
I make the movement tough.
I may be—
static, sliding or rolling
but whenever two surfaces are in motion
I am always there,
Tell me who I am!
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Exercises
1. Fill in the blanks.
(a) Friction opposes the _____________ between the surfaces in contact
with each other.
(b) Friction depends on the _____________ of surfaces.
(c) Friction produces __________.
(d) Sprinkling of powder on the carrom board ________ friction.
(e) Sliding friction is ___________ than the static friction.
2. Four children were asked to arrange forces due to rolling, static and sliding
frictions in a decreasing order
. Their arrangements are given below.
Choose the correct arrangement.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling
3. Alida runs her toy car on dry marble floor, wet marble floor, newspaper
and towel spread on the floor. The force of friction acting on the car on
different surfaces in increasing order will be
(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper
4. Suppose your writing desk is tilted a little. A book kept on it starts sliding
down. Show the direction of frictional force acting on it.
5. You spill a bucket of soapy water on a marble floor accidently. Would it
make it easier or more difficult for you to walk on the floor? Why?
6. Explain why sportsmen use shoes with spikes.
7. Iqbal has to push a lighter box and Seema has to push a similar heavier
box on the same floor. Who will have to apply a larger force and why?
8. Explain why sliding friction is less than static friction.
9. Give examples to show that friction is both a friend and a foe.
10. Explain why objects moving in fluids must have special shapes.
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Extended Learning — Activities and Projects
1. What role does friction play in the sport of your choice? Collect
some pictures of that sport in action where friction is either
supporting it or opposing it. Display these pictures with proper
captions on the bulletin board of your classroom.
2. Imagine that friction suddenly vanishes. How would life be affected.
List ten such situations.
3. Visit a shop which sells sports shoes. Observe the soles of shoes
meant for various sports. Describe your observations.
4. A toy to play with:
Take an empty match box. Take out its tray. Cut a used refill of a ball
pen of the same width as the tray as shown in the figure below. Fix
the refill with two pins on the top of the tray as shown in Fig. 9.18.
Make two holes on the opposite sides of the tray. Make sure that the
holes are large enough to allow a thread to pass through them easily.
Take a thread about a metre long and pass it through the holes as
shown. Fix beads at the two ends of the thread so that it does not
come out. Insert the tray in the outer cover of the matchbox.
Suspend the match box by the thread. Leave the thread loose.
The match box will start falling down due to gravity. Tighten the
thread now and observe what happens.
Explain your observation. Can you relate it to friction?
Fig. 9.18
You can read more on the related topic on the following websites :
l http://www.school-for-champions.com/science/friction.htm
l http//hyperphysics.phy-astr.gsu.edu/hbase/firct2.html
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